The structure indicated is the minor calyx of the kidney.

The kidney consists of an outer cortex and an inner medulla. The cortex projects into the kidney dividing the medulla into triangular shaped renal pyramids, whose apices are surrounded by a minor calyx (calices = plural). The minor calices join to form a major calyx, which in turn unite to form the renal pelvis which exit the kidney to form the ureters. The calyx system of the kidney serves to drain urine. The smooth muscle walls of the calices are capable of peristalsis, which helps to propel urine through the kidneys, out through the renal pelvis and into the bladder via the ureters.

Learn all about the anatomy of the internal structure of the kidney in this video tutorial.